1. Field of the Invention
The invention relates to layers of an energy absorbing, transparent, high optical quality, plastic material suitable for use in laminated glass comprised of a sheet of glass and at least one layer of the plastic material, e.g., vehicle windshields.
2. Discussion of the Background
Layers of plastic materials having energy-absorbing properties have already been described in European Patent Publication EP-0 133 090. These layers are formed during a continuous process involving the reactive pouring, on a horizontal flat base, a reactive mixture composed of an isocyanate constituent and an active hydrogen constituent. The active hydrogen constituent is especially a polyhydric alcohol constituent. The isocyanate constituent contains at least one aliphatic or cyclo-aliphatic diisocyanate or a diisocyanate prepolymer. This constituent has a viscosity of less than approximately 5 Pas at +40.degree. C. The polyhydric alcohol constituent contains at least one long difunctional polyhydric alcohol having a molecular weight of between 500 and 4,000, and at least one short diol acting as a chain-elongating agent. The term "reactive pouring" means pouring, in the form of a layer or film, a fluid mixture of constituents in the monomer or prepolymer state, followed by the heat polymerization of this mixture.
The proportions of the polyurethane constituents are chosen so as to obtain, preferably, a stoichiometrically-balanced system. That is, the ratio of the NCO equivalent groups supplied by the diisocyanate constituent to the OH equivalent groups supplied by the polyhydric alcohol(s) or the short diols, is approximately 1.
Appropriate diisocyanates are chosen, in particular, from among the following difunctional aliphatic isocyanates: hexamethylenediisocyanate (HMDI), 2,2,4-trimethyl-1, 6-hexanediisocyanate (TMDI), bis-4-isocyanatocylcohexylmethane, 2,2-bis(4-isocyanatocyclohexyl propane (IPDI), m-xylylenediisocyanate (XDI), m and p-tetramethylxylylenediisocyanate (m- and p- TMXDI), trans-cyclohexane-1,4-diisocyanate (CHDI), 1,3-(diisocyanatomethyl) cyclohexane (hydrogenated XDI). IPDI and IPDI containing urea functions are preferred.
Heat polymerization of these preferred, IPDI-based layers uses a thermal cycle normally consisting of at least 20 minutes at a temperature of approximately 120.degree. C., which thus involves a long polymerization tunnel in the case of continuous reactive pouring. Efforts have not been made to shorten this polymerization cycle, for reasons of cost and line length.
One of the most widely-used diisocyanates in the manufacture of thermoplastic polyurethane layers, which is also the most frequently cited in the literature, for example in U.S. Pat. No. 3,620,905, is bis-4-isocyanatocyclohexylmethane, marketed, for example, under the names Hylene W or Desmodur W. Reactivity is one of the reasons for its extensive use. However, tests conducted by the inventors have shown that its use in a reactive pouring process under normal pouring temperature conditions, i.e., of approximately 40.degree., to manufacture a polyurethane layer having energy-absorbing properties, always led to a layer exhibiting a haze. Considered from an optical perspective, this made the layer unsatisfactory.
The haze can be avoided by pouring the reactive mixture at a much higher temperature, for example of about 120.degree. or more. However, at that temperature polymerization occurs too rapidly and takes place prematurely in the pouring head. As a consequence, optical defects, such as striae, form on the layer.